1. Field Of The Invention
The present invention relates to a method of fabricating an insulated-gate field effect transistor (IG-FET) having a channel with a relatively narrow width, i.e., 2 microns or less, and in which lateral diffusion of field impurities doped to form a so-called "channel stopper" occurs, and more specifically, to a method of fabricating an IG-FET which lowers the threshold voltage to a desired level even when the channel is entirely filled with the laterally diffused field impurities.
2. Description Of The Related Art
The reduction of the scale of semiconductor devices, such as transistors, is a major issue for semiconductor integrated circuit (IC) manufacturers with regard to increasing packing density and operating speed, and further to reducing power consumption of semiconductor ICs. For increases in the operating speed, the channel length of IG-FETs or metal-insulator-semiconductor (MIS) FETs incorporated in integrated circuits have been decreased to a dimension near 1.5 microns; further decreases in the channel length are continually being devised. At the same time, reductions in the width of the channel have been provided to decrease power consumption in IG-FETs which do not require high speeds or large current capacities. After the channel width of an IG-FET has been decreased to a dimension near 2 microns or less, the threshold voltage of the FET rises as the width decreases. The threshold voltage increase associated with a narrow channel width is considered to be a result of structure of narrow channel IG-FETs. (W. A. Noble et al. "Narrow Channel Effect in Insulated Gate Field Effect Transistors" Proc. of IEEE 1976 International Electron Devices, pp. 582-586).
Impurities diffused into the channel from the surrounding region, particularly impurities which are doped in the field to establish a desired field inversion threshold voltage, are also considered to cause an increase in the threshold voltage when the channel width is reduced. Such impurities are selectively doped into the field around the active region (transistor region) in which the FET is formed. The impurities are diffused downwardly in the semiconductor substrate into a region below the field oxide layer; the downward diffusion occurs during the heat processing performed to form a field oxide layer. The region doped with such impurities is referred to as a channel stopper. However, a lateral diffusion of the impurities into the active region occurs--the degree of lateral diffusion corresponding to the depth of the downward diffusion. This lateral diffusion causes the impurities forming the channel stopper to encroach upon the active region, and since these impurities have the same conductivity type as that of the portion of the substrate or well in which the associated IG-FET is formed, the threshold voltage of a FET increases if the impurity concentration in the channel of the FET is increased by laterally diffused impurities.
A technology known as a channel doping process is employed to adjust the threshold voltage of an IG-FET to a desired level. By selecting the conductivity type and the dosage of the impurities, the FET can be characterized as a depletion or enhancement type having the desired conductivity type and threshold voltage. Channel doping is sometimes performed to a relatively large depth, with respect to the thickness of the corresponding semiconductor device, to create a well in the substrate. The impurity concentration in the well may be controlled to establish a desired threshold voltage of an IG-FET formed therein.
In the channel doping process, a relatively low dosage of impurities is concurrently diffused into the remaining portions of IG-FETs, having the same conductivity type, formed on the semiconductor substrate. The concentration of the impurities provided by lateral diffusion, however, is higher than the concentration of impurities provided by channel doping. Accordingly, if an IG-FET having a narrow channel width is formed on the same semiconductor substrate as an IG-FET having a relatively wider channel width, the threshold voltage increase in a narrow channel width IG-FET due to the lateral diffusion cannot be controlled by conventional channel doping technology. Thus, using existing technology, a narrow channel IG-FET having a channel width of 2 microns or less cannot be put into practical use, much less incorporated in an IC chip including conventional wider channel IG-FETs.